Wireless communication systems, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. Wireless service providers continually try to create new markets for wireless devices and to expand existing markets by making wireless devices and services cheaper and more reliable. The price of end-user wireless devices, such as cell phones, pagers, PCS systems, and wireless modems, has been driven down to the point where these devices are affordable to nearly everyone and the price of a wireless device is only a small part of the end-user's total cost. To continue to attract new customers, wireless service providers concentrate on reducing infrastructure costs and operating costs, and on increasing handset battery lifetime, while improving quality of service in order to make wireless services cheaper and better.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
CDMA technology is used in wireless computer networks, paging (or wireless messaging) systems, and cellular telephony. In a CDMA system, mobile stations and other access terminals (e.g., pagers, cell phones, laptop PCs with wireless modems) and base stations transmit and receive data on the same frequency in assigned channels that correspond to specific unique orthogonal codes. For example, a mobile station may receive forward channel data signals from a base station that are convolutionally coded, formatted, interleaved, spread with a Walsh code and a long pseudo-noise (PN) sequence. In another example, a base station may receive reverse channel data signals from the mobile station that are convolutionally encoded, block interleaved, modulated by a 64-ary orthogonal modulation, and spread prior to transmission by the mobile station. The data symbols following interleaving may be separated into an in-phase (I) data stream and a quadrature (Q) data stream for QPSK modulation of an RF carrier. One such implementation is found in the TIA/EIA-95 CDMA standard (also known as IS-95). Another implementation is the TIA/EIA-2000 standard (also known as IS-2000).
The current generation of cellular phones is used primarily for voice conversations between a subscriber device (or wireless device) and another party through the wireless network. A smaller number of wireless devices are data devices, such as personal digital assistants (PDAs) equipped with cellular/wireless modems. Because the bandwidth for a current generation wireless device is typically limited to a few tens of kilobits per second (kbps) , the applications for the current generation of wireless devices are relatively limited. However, this is expected to change in the next (or third) generation of cellular/wireless technology, sometimes referred to as “3G” cellular/wireless, where much greater bandwidth will be available to each wireless device (i.e., one hundred twenty five thousand bits per second (125 kbps) or greater). The higher data rates will make Internet applications for wireless devices much more common. For instance, a 3G cellular telephone (or a PC with a 3G cellular modem) may be used to browse web sites on the Internet, to transmit and receive graphics, to execute streaming audio or video applications, and the like. A much higher percentage of the wireless traffic handled by 3G cellular systems will be Internet protocol (IP) traffic and a lesser percentage will be traditional voice traffic.
Real-time streaming of multimedia content over Internet protocol (IP) networks has become an increasingly common application in recent years. As noted above, 3G wireless networks will provide streaming data (both video and audio) to wireless devices for real time applications. A wide range of interactive and non-interactive multimedia Internet applications, such as news on-demand, live TV viewing, video conferencing, live radio broadcasting (such as Broadcast.com) , and the like, will provide “real time” data streaming to wireless devices. Unlike a “downloaded” video file, which may be retrieved first in “non-real” time and viewed or played back later, real time (or streaming) data applications require a data source to encode and to transmit a streaming data signal over a network to a receiver, which must decode and play the signal (video or audio) in real time.
The CDMA 2000 Air Interface Standard provides a rescue channel for the “rescue” of a mobile station that has lost it traffic channel. The term “rescue” refers to the re-establishment of communications between the mobile station and a base station on a rescue channel. To understand the operation of the rescue channel, consider a mobile station (MS) that is communicating with a base station controller (BSC) on an active call. During the active call the mobile station sends a Pilot Strength Measurement Message (PSMM) to the base station controller. The PSMM indicates to the base station controller the identity of local base stations that are near the mobile station (referred to as “neighbor” base stations). The mobile station keeps a neighbor list identifying the neighbor base stations.
If a neighbor base station is capable of providing a rescue channel for the mobile station, the base station controller notifies the mobile station by sending the mobile station an extended neighbor list update message. The extended neighbor list update message sends to the mobile station (1) the rescue channel Walsh codes, and (2) the Quasi-Orthogonal function for the neighbor base station. These codes are specially reserved codes on the neighbor base station. These codes are used only in case a rescue is attempted. In this manner a rescue channel is reserved for the use of the mobile station.
If during an active call the mobile station receives two or more bad frames, the mobile station will shut down its transmitter and wait until communication conditions improve. The time during which communications are interrupted is referred to as a “fade” period. After the communication conditions have improved, the mobile station sends an Extended Pilot Strength Measurement Message (Extended PSMM) to the base station controller. During the “fade” period if the mobile station notices that the pilot signal of a neighbor base station in the neighbor list has grown in strength, the mobile station may automatically promote that pilot signal into its active set. The promotion of the neighbor base station is communicated from the mobile station to the base station controller in the Extended PSMM. The Extended PSMM indicates to the base station controller that the mobile station would like to attempt a rescue on the promoted neighbor base station.
The promoted neighbor base station (now the “rescue” base station) begins transmitting on the previously reserved rescue channel. At the same time the rescue base station is listening for the mobile station on the mobile station's reverse traffic channel. When the rescue base station acquires the mobile station, a regular handoff may be initiated in order to move the mobile station of the rescue channel.
The presently existing rescue channel feature in the CDMA 2000 Air Interface Standard does not provide for rescue channel communications between base stations in a wireless communication system. Therefore, there is a need in the art for a system and method that is capable of providing rescue channel communications between base stations in a wireless communication system. In particular, there is a need for a system and method that is capable of providing rescue channel messages in a proper format to enable base stations in a wireless communications system to coordinate and carry out rescues of mobile station communications using the rescue channel feature.